In various process industries, particularly the chemical process industries, various liquids, gases or slurries are maintained under vacuum or under pressure, either in vessels or in flow conduits, and the pressure values must be monitored. For this purpose, suitable gauges or recording instruments are mounted on the process vessels or on the flow lines or are remotely coupled thereto, to sense, indicate and record the existing pressure values.
In most chemical processes, as well as in many other process industries, it is both necessary and desirable to isolate the pressure sensing means of the recording instrument from the abrasive or corrosive or gumming action of the process material so that the sensing means will not be subjected to undue deterioration, injury or wear. To accomplish this isolation, it is common practice to provide a flange member or similar coupling unit having a cavity in communication with the pressure recording instrument. This instrument flange member is secured in abutting relationship to a process flange member which has a cavity in communication with the pressure fluid to be monitored. A flexible diaphragm of metal, rubber, or synthetic resin serves to partition the instrument cavity from the process fluid cavity and is secured between the coupling or flange members at its marginal positions. A suitable compressive sealing gasket overlays the secured marginal portion of the diaphragm to effect the necessary seal on the margin of the diaphragm when the coupling members are bolted together or otherwise secured in operational abutment.
One side of the flexible diaphragm is thereby exposed to the pressure of the process fluids. The cavity in the instrument flange on the other side of the flexible diaphragm is filled with light oil or other suitable hydraulic fluid for communication with the pressure sensing means. The central portion of the flexible diaphragm is displaced or expanded in response to pressure fluctuations from the process fluid cavity and transmits these changing pressure values through the hydraulic fluid medium in the instrument flange cavity for sensing by recording gauge or instrument.
After some period of use, whose duration depends greatly upon the character of the process fluid, the process fluid cavity as well the process side of the flexible diaphragm will have gathered or accumulated a sufficient amount or quantity of deposits of sludge or precipitates or other solids from the process fluids to which it is exposed, so that cleaning of the process cavity and the process flange becomes necessary. The frequency of this cleaning can be diminished by the use of various forms of "flush-out" arrangements and fittings in a manner known to the art, but eventually a more thorough cleaning becomes mandatory and requires disassembly of the unit for access to the interior of the process flange member. Inasmuch as the marginal portion of the diaphragm is clamped between the two flange members, it no longer has any securement when the flange members are uncoupled for disassembly and cleaning. Its seal with the hydraulic instrument fluid is broken when the flanges are disassembled. This ordinarily results in the loss of all or a good part of this hydraulic fluid which must then be replaced when the unit reassembled. The replacement of the hydraulic fluid is not only a tedious and time-consuming task, but can also result in the necessity for recalibrating the unit as a result of there being a greater or lesser quantity of the replacement hydraulic fluid than was contained in the unit before the disassembly. The sealing gaskets will ordinarily have acquired a permanent set which makes it necessary for the gasket to be replaced after the cleaning operation has been completed and the unit is to be reassembled. If the gasket is not replaced, it is quite likely that an imperfect seal between the coupling members will result on the reassembly of the unit and that leakage will occur.
A prior art patent that has attempted to address this problem is U.S. Pat. No. 3,645,139 issued on February 29, 1972, to the present inventor. This patent describes a flexible diaphragm having an annular groove around its periphery for the purpose of defining a resilient shoulder. The diaphragm is mounted in a coupling member which communicates with the instrument and is snapped into detachable securement over a projection or shoulder of the coupling member, which fits into the annular groove. The groove may carry an O-ring or similar compressible seal to effect the sealing engagement between the seat of the groove and the inserted shoulder. In actual practice, this diaphragm was manufactured from TEFLON material and each of the grooves were machined into the TEFLON material. This self-retaining diaphragm seal has proven, in use, to be very effective when it is necessary to uncouple the flange members for cleaning or other attention. The diaphragm is retained by the instrument flange member in sealing engagement with the hydraulic fluid without any reliance upon any clamping securement between the coupling members. Importantly, however, it was found to be very difficult to manufacture the invention of U.S. Pat. No. 3,645,139 from metallic materials. The many desirable qualities of sensitivity and durability are found in TEFLON but are generally considered incompatible qualities in metal. Additionally, the machining processes required to manufacture the attachment mechanism of this TEFLON diaphragm were found virtually impossible for metal diaphragms.
In designing metallic pressure-responsive diaphragms, two generally incompatible characteristics are sought to be maximized sensitivity and durability. Sensitivity as measured by the force or pressure necessary to temporarily deform or deflect the diaphragm directly affects the accuracy of the pressure measurement. Durability affects both accuracy and practicality; if the diaphragm lacks durability, it can become permanently deformed, causing inaccuracy or rupture, thereby causing damage to the recording mechanism. The less durable a diaphragm is, the more often it must be replaced, causing costly and undesirable "downtime".
Increased durability is achieved at the expense of sensitivity. To achieve accuracy coupled with durability, diaphragm designers have focused on structures whose effective surface area remains substantially constant over a moderately wide range of deflection and whose surfaces contain corrugations or convolution to permit deflection of the diaphragm with minimal stress on the diaphragm fibers. The diaphragm material must, of course, be thin enough to flex easily, yet thick enough to resist corrosion and permanent deformation or rupture. When made of metal, a diaphragm for the pressure recording instrument is typically 0.004 to 0.005 inches thick and about two to four inches in diameter.
Whatever the exact thickness or composition of the diaphragm material, a flat diaphragm is subject to two distinct types of tensile stress as the diaphragm is deflected under pressure. One of these is radial stress; the other is circumferential stress (commonly called "hoop stress"). These stresses occur because, as pressure is applied to one side of a flexible diaphragm, the diaphragm will flex or "dome out" in the opposite direction, thereby assuming a convex or parabolic profile. The diaphragm material will be stretched both along its diameter (radial stress) and circumferentially. It is evident that the surface area of the diaphragm in its domed position is greater than the surface area of the diaphragm in its rest position. This enlargement of the diaphragm surface stretches the diaphragm fibers circumferentially thereby creating "hoop stress".
U.S. Pat. No. 4,375,182, issued on March 1, 1983, to the present inventor, is entitled "Ultra-Sensitive Diaphragm With Dual Stress-Relief Structures." It is the object of this invention to overcom both the limitations of a trough-shaped annular corrugation and the stiffening-neutralizing effect of spokes by means of a novel diaphragm structure that has a plurality of radially extending corrugations with multiple annular corrugation in an integrated structure that retains, rather than disrupts, the annular waveform configuration and thereby substantially increases diaphragm sensitivity and durability through the simultaneous reduction of radial and circumferential tensile stresses. By using the very thin material, this patented diaphragm seal offered significant advantages over prior art metallic seals. Metallic seals are ultimately required, over TEFLON, in various process industries. It has been found that TEFLON seals allow chlorine molecules to seep through. There are other processes, using chlorine derivatives or compounds, which cause similar problems for TEFLON diaphragms. As a result, in the chemical and process industry, there is a real need for the use of metallic diaphragm seals.
In the past, it has been necessary to fixedly mount the diaphragm of U.S. Pat. No. 4,375,182 to the instrument flange by welding the diaphragm at the periphery to the instrument flange. As a result, the desirable features of the self-retaining diaphragm seal of U.S. Pat. No. 3,645,139 were impossible to incorporate with the metal diaphragm. The process of welding the diaphragm is a time consuming and expensive process. Additionally, if it becomes necessary to replace the metallic diaphragm, the associated instrument flange must also be replaced.
It is an object of the present invention to provide an ultra-sensitive metallic diaphragm with a snap-in self-retaining quality.
It is an other object of the present invention to provide such a diaphragm that maintains an effective seal between the process flange in the instrument flange.
It is a further object of the present invention to provide such a diaphragm that offers the sensitivity of a thin metallic diaphragm.
It is still a further object of the present invention to provide a diaphragm seal that is relatively inexpensive to manufacture and assemble.
These and other objects and advantages of the present invention will become apparent from the reading of the attached specification and appended claims.